Plastics, more particularly thermoplastics, have been used for some considerable time. Their use, however, entails a number of disadvantages. On account of their softness they are highly susceptible to scratches. This is a very great disadvantage more particularly in the case of visible and/or exposed plastics parts. Many of these plastics are transparent and are therefore frequently used as a substitute for glass, in the form of windows or covers. In these applications scratches are likewise a very great disadvantage, since light may be deflected by the scratches and, as a result, these windows may become unclear to cloudy.
In order to eliminate these disadvantages of such plastics they have for some considerable time been coated with scratch-resistant silicone compositions. Silicone compositions of this kind are applied to the plastics and baked. These thermally cured silicone coatings are also known to the English-speaking skilled worker by the term “silicone hard coats”. Coatings of this kind are disclosed, for example, by U.S. Pat. No. 5,041,313, U.S. Pat. No. 4,624,870, and EP 0 570 165 A2, or G. Medford et al. in “The Next Generation in Weatherable Hardcoats for Polycarbonate”, International Coatings for Plastics Symposium, Jun. 4-6, 2001, Troy, Mich. In many cases, however, shaped parts comprising such plastics treated with thermally cured silicone coatings have to be joined to other shaped parts. On account of the known advantages of adhesive bonding technology it is desirable to bond or seal these parts. It has emerged, however, that coated parts of this kind are very difficult to bond or seal with typical adhesives or sealants, respectively, more particularly with one-component polyurethane adhesives or sealants.
EP 1 382 625 A1 solves this problem through the use of a specific isocyanate-containing primer featuring good adhesion to plastics such as poly(methyl methacrylate) or polycarbonate that have polydimethylsiloxane-based coatings (PDMS-PMMA or PDMS-PC). A disadvantage of this, however, is that an additional step, namely the application of the primer, is needed on the manufacturing line where shaped parts of this kind are employed. Even more disadvantageous, however, is the fact that the necessary flashing-off, i.e., the time between application of the primer and application of the adhesive or sealant, gives rise to a waiting time, which for a continuous manufacturing operation necessitates, for example, an intermediate store. Furthermore, the solvents that are typically present in primers frequently lead to corrosive stress cracking.